In response to the adverse effects of the fixed rate pacing intrinsic to early implantable pacemakers, "rate-responsive" pacemakers were developed which can automatically adjust the patient's heart rate in accordance with metabolic demands. Similarly, implantable cardioverter defibrillators (ICDs) that include pacing circuitry also may, and preferably do, pace in a rate-responsive manner. An implanted rate-responsive pacemaker (or ICD having rate-responsive pacing capabilities) typically operates to maintain a predetermined minimum heart rate when the patient is engaged in physical activity at or below a threshold level, and gradually increases the maintained heart rate in accordance with increases in physical activity until a maximum rate is reached. Thus, such rate-responsive pacemakers typically include processing circuitry that correlates measured physical activity to an appropriate heart rate. In many rate-responsive pacemakers, the minimum heart rate, maximum heart rate and the transition rates between the minimum and maximum heart rates are parameters that may be telemetrically adjusted to meet the needs of a particular patient.
Most rate-responsive pacemakers employ sensors that transduce mechanical forces associated with physical activity to determine the level of metabolic need of a patient, relying upon the clinical association of body motion with increasing levels of exercise. These activity sensors generally contain a piezoelectric transducing element which generates a measurable electrical potential when a mechanical stress resulting from physical activity is experienced by the sensor. Thus, by analyzing the signal from a piezoelectric activity sensor, a rate-responsive pacemaker can determine how frequently pacing pulses should be applied to the patient's heart.
Another physiological sensor frequently used in rate-responsive pacemakers are respiration sensors. Such sensors may be employed to measure respiratory rate (RR), tidal volume (TV) or the product of these two parameters, minute ventilation (MV). Each of these parameters increases in proportion to changes in carbon dioxide production. Minute ventilation-sensing, rate-adaptive pacing systems have been demonstrated to provide rate modulation that is closely correlated with oxygen consumption in most patients implanted with these devices. See, CARDIAC PACING, edited by Kenneth A. Ellenbogen, Blackwell Scientific Publications, Cambridge, Mass. (1992), page 94.
Minute ventilation is generally estimated by frequent measurements of transthoracic impedance between an intracardiac lead and the pulse generator case using a tripolar system. Transthoracic impedance increases with inspiration and decreases with expiration. Thus, by measuring the frequency of respiration-related fluctuations in impedance (correlated with respiratory rate) and the amplitude of those excursions (correlated with tidal volume), the estimated minute ventilation can be calculated. (CARDIAC PACING, referred to above, at page 94.)
It is well established that a myriad of physiological processes demonstrate a rhythmic variation during a twenty-four hour period. Such daily variations are referred to as circadian or diurnal variations. The intrinsic cardiac rhythmicity of both normal and diseased hearts is subject to circadian variations. For example, minimum heart rate decreases during sleep. See, Djordjevic, M. et al., "Circadian Variations of Heart Rate and STIM-T Interval: Adaptation for Nighttime Pacing," PACE, 12:1757-1762 (1989). While rate-responsive pacemakers are able to pace according to the presently measured metabolic need of the patient, many are set with minimum pacing rates that are not adjusted according to such circadian fluctuations. In fact some pacemakers are programmed with a single minimum pacing rate as high as 80 bpm, because the rate does not change. However, recently researchers have shown that sustaining such rates around the clock may induce cardiac performance that is consistent with heart failure. See, Chew, et al., "Overnight heart rate and cardiac function in patients with dual-chamber pacemakers," PACE 19:822-828 (1996).
Thus, a number of cardiac pacemakers have been developed to sense stages in a patient's circadian rhythm or activity level and to alter the output of pacing pulses in response thereto. For example, U.S. Pat. Nos. 4,922,930 and 5,143,065, both issued to Adkins et al., disclose a cardiac pacemaker which can vary the rate of pacing pulses in accordance with a wake-sleep cycle based on a model having multiple time periods. Each period has a specific duration, which periods are maintained with a real-time clock located within the pacemaker, and a predicted minimum physiologic need for the patient is correlated to each time period.
Similarly, U.S. Pat. No. 4,945,909 to Fearnot et al. discloses a pacemaker that paces at a rate defined within a range having variable upper and lower rate limits. These limits change in response to patient activity sensed by the pacemaker. U.S. Pat. No. 5,300,092 to Schaldach also discloses a cardiac pacemaker which can vary the rate of pacing pulses in response to the patient's sensed activity. U.S. Pat. No. 5,476,483 to Bornzin et al. discloses a cardiac pacemaker that varies a base pacing rate of a predetermined transfer function according to sensed activity levels, and U.S. Pat. No. 5,514,162, also issued to Bornzin et al., describes a method for automatically determining the slope of a transfer function that is used by the pacemaker to determine appropriate heart rates in accordance with metabolic demands. Each of U.S. Pat. Nos. 5,476,483 and 5,514,162 are hereby incorporated by reference, in their entirety.
These pacemakers can produce a cardiac rhythm that more closely mimics a natural rhythm than pacemakers that do not change their output in response to activity levels or to stages in a patient's circadian rhythm, but the degree to which they mimic the patient's natural rhythm varies.
Most patients would benefit, then, from a pacemaker which could vary its output to mimic a natural rhythm even more closely and reliably. Additionally, as a patient's heart or lifestyle changes over time, it would be advantageous to have a pacemaker which can vary its output in response to a patient's circadian rhythm in a manner that is selectable according to the patient's needs. Still further, it is desirable that any new methods of adjusting pacing rates to mimic circadian rhythms be easily applied to existing pacemakers having the ability to sense circadian biorhythms.